drm: crtc: integer overflow in drm_property_create_blob()

The size here comes from the user via the ioctl, it is a number between
1-u32max so the addition here could overflow on 32 bit systems.

Fixes: f453ba0 ('DRM: add mode setting support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Cc: stable@kernel.org # v4.2
Signed-off-by: Dave Airlie <airlied@gmail.com>

drivers/gpu/drm/drm_crtc.c

@@ -4105,7 +4105,7 @@ drm_property_create_blob(struct drm_device *dev, size_t length,
 	struct drm_property_blob *blob;
 	int ret;
 
-	if (!length)
+	if (!length || length > ULONG_MAX - sizeof(struct drm_property_blob))
 		return ERR_PTR(-EINVAL);
 
 	blob = kzalloc(sizeof(struct drm_property_blob)+length, GFP_KERNEL);